Embodiments of the invention relate generally to medical imaging, and more specifically, to a stand-alone magnetic resonance (MR) imaging system or hybrid MR and positron emission tomography (PET) system incorporating a surface stationary RF coil structure that provides patient support while reducing image degradation.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged photons known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
In combination PET-MR systems, it is desirable to have minimum mass in the region of the PET detector in order to provide for optimum image acquisition. That is, while in a standalone MR system the structure and mass of components within the bore has no effect on image acquisition and image quality, such is not the case in a PET-MR system—as the mass in the PET detector region attenuates gamma rays, which reduces PET signal to the detectors and degrades image quality (IQ).
To minimize IQ attenuation, the design of both stationary and moving objects that are required to be in or go through the PET detector region should therefore be such so as to minimize mass in the PET detector region. Such stationary objects can include, for example, a patient positioning structure including a bridge positioned within the bore that extends through a length of the imaging system and a surface stationary radio frequency (RF) coil structure (e.g., posterior coil), while the moving object may comprise a cradle that supports the patient and translates along the bridge to move the patient through the imaging system.
With specific regard to the stationary posterior RF coil structure, the RF coil structure is positioned at the center of the MR and PET field of view (FOV) to acquire MR image data from the patient. The stationary posterior RF coil structure includes a number RF elements on its surface facing the bottom of the cradle that are in close proximity to the patient anatomy during an MR imaging scan, with a small gap being present between the RF coils and the cradle that keeps the cradle from rubbing on the coil elements as the cradle moves in and out of the magnet bore. Typically, any electronics associated with the stationary posterior RF coil structure, such as decoupling boards, feedboards, mux boards, and baluns for example, are positioned immediately adjacent the RF coil elements, and are thus also positioned at the center of the MR and PET FOV, thereby adding mass and additional components in the region of the PET detector and potentially reducing image quality.
Apart from PET image attenuation considerations, it is also recognized that the structure of the stationary posterior RF coil structure can affect MR image quality in a stand-alone MR imaging system or hybrid PET-MR system. That is, it is desirable to enable placement of the RF coil elements of the coil structure in close proximity with the patient anatomy so as to enable good MR image quality. In so positioning the RF coil elements, it is also desirable to provide a constant and uniform gap between the entire set of RF elements in the coil structure and a patient cradle surface. Existing designs of the stationary posterior RF coil structure provide a flat configuration or construction that lacks the curvature that is normally found on the patient cradle, such that existing posterior coil structure designs fail to position the RF coil elements in close proximity to the patient anatomy or provide such a uniform gap between the coil elements and the patient surface on the cradle.
It would therefore be desirable to provide a stationary posterior RF coil structure for use in a PET-MR system that helps in reducing the degradation of image quality by minimizing mass in the PET detector FOV. It would also be desirable for the stationary posterior RF coil structure to provide improved image quality without compromising on patient support functionalities and serviceability. It would still further be desirable to for the stationary posterior RF coil structure to include features thereon that enable placement of the RF coil elements for good proximity with patient anatomy, with such features following the contour of the cradle sides and enabling the coil elements to more closely view the patient anatomy where the sides of the patient move away from the horizontal cradle surface.